Integrated strained stacked nanosheet FET

ABSTRACT

Methods of forming a semiconductor device include forming stress liners in contact with both ends of a fin of alternating channel material and sacrificial material layers. The stress liners exert a stress on the fin. The sacrificial material is etched away from the fin, such that the layers of the channel material are suspended between the stress liners. A gate stack on the suspended layers of channel material.

BACKGROUND Technical Field

The present invention generally relates to semiconductor devices and,more particularly, to strained channel field effect transistors.

Description of the Related Art

As semiconductor fabrication processes produce increasingly smallfeatures, nanowires and nanosheets are being used to further the scalingof complementary metal-oxide semiconductor (CMOS) devices, and fieldeffect transistors (FETs) in particular.

Strain engineering is used in CMOS fabrication to boost deviceperformance. Putting a compressive on a p-type CMOS transistor or atensile strain on an n-type CMOS transistor can enhance electron (orhole) mobility in the transistor's channel and thereby improveconductivity of the device. Various techniques may be used to inducestrain in a semiconductor structure.

However, when, for example, a strained semiconductor fin is cut tolength, the induced strain can relax at the fin ends. The loss of strainat fin ends can cause device degradation and variation.

SUMMARY

A method for forming a semiconductor device includes forming stressliners in contact with both ends of a fin of alternating channelmaterial and sacrificial material layers. The stress liners exert astress on the fin. The sacrificial material is etched away from the fin,such that the layers of the channel material are suspended between thestress liners. A gate stack on the suspended layers of channel material.

A method of forming a semiconductor device includes forming a dielectricfill around a fin of alternating channel material and sacrificialmaterial layers. Stress liners are formed in contact with both ends ofthe fin after forming the dielectric fill. The stress liners exert astress on the fin. The sacrificial material is etched away from the fin,such that the layers of the channel material are suspended between thestress liners. A gate stack is formed on the suspended layers of channelmaterial.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a step in forming an integrated,strained, stacked nanowire field effect transistor (FET) in accordancewith the present principles;

FIG. 2 is a top-down diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 3 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 4 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 5 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 6 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 7 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 8 is a perspective diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 9 is a cross-sectional diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 10 is a cross-sectional diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 11 is a cross-sectional diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples;

FIG. 12 is a cross-sectional diagram of a step in forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples; and

FIG. 13 is a block/flow diagram of a method of forming an integrated,strained, stacked nanowire FET in accordance with the presentprinciples.

DETAILED DESCRIPTION

Embodiments of the present invention provide strain at the ends ofnanosheets and nanowires by providing a stress liner at opposite ends ofthe devices. For p-type field effect transistors (pFETs), a compressivestress liner is used, while for n-type field effect transistors (nFETs),a tensile stress liner is used. The liner is deposited before thenanowires or nanosheets are freed from their fins.

Referring now to FIG. 1, a step in fabricating an end-stressedtransistor device is shown. The figure shows asemiconductor-on-insulator substrate with a first semiconductor layer106 on an insulator layer 104 and a bulk substrate 102 below them. Thebulk semiconductor substrate 102 may be a silicon-containing material.Illustrative examples of silicon-containing materials suitable for thebulk-semiconductor substrate include, but are not limited to, silicon,silicon germanium, silicon germanium carbide, silicon carbide,polysilicon, epitaxial silicon, amorphous silicon, and multi-layersthereof. Although silicon is the predominantly used semiconductormaterial in wafer fabrication, alternative semiconductor materials canbe employed, such as, but not limited to, germanium, gallium arsenide,gallium nitride, silicon germanium, cadmium telluride and zinc selenide.

It is specifically contemplated that the insulator layer 104 can beformed from silicon dioxide, but it should be understood that anyappropriate dielectric material may be used instead. A dielectricmaterial may be low-k (i.e., having a dielectric constant k that islower than that of silicon dioxide) or high-k (i.e., having a dielectricconstant k that is higher than that of silicon dioxide. Exemplaryalternative low-k dielectrics include SiCOH, carbon-doped oxides, andundoped silicon glass. Exemplary alternative high-k dielectrics includehafnium oxides, hafnium silicates, titanium oxides,barium-strontium-titantates, and lead-zirconate-titanates.

The first semiconductor layer 106 is the first layer in a stack ofsheets of alternating materials. The stack includes first semiconductorlayers 107 and second semiconductor layers 108. Although it isspecifically contemplated that the first semiconductor layers 107 may beformed from silicon germanium and that the second semiconductor layers108 may be formed from silicon, it should be understood that anyappropriate materials may be used instead, as long as the twosemiconductor materials have etch selectivity with respect to oneanother. As used herein, the term “selective” in reference to a materialremoval process denotes that the rate of material removal for a firstmaterial is greater than the rate of removal for at least anothermaterial of the structure to which the material removal process is beingapplied. The alternating layers 107/108 can be deposited by anyappropriate mechanism. It is specifically contemplated that the layersmay be epitaxially grown from one another, but alternate depositionprocesses, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), or gas cluster ion beam(GCIB) deposition, are also contemplated.

CVD is a deposition process in which a deposited species is formed as aresult of chemical reaction between gaseous reactants at greater thanroom temperature (e.g., from about 25° C. about 900° C.). The solidproduct of the reaction is deposited on the surface on which a film,coating, or layer of the solid product is to be formed. Variations ofCVD processes include, but are not limited to, Atmospheric Pressure CVD(APCVD), Low Pressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), andMetal-Organic CVD (MOCVD) and combinations thereof may also be employed.In alternative embodiments that use PVD, a sputtering apparatus mayinclude direct-current diode systems, radio frequency sputtering,magnetron sputtering, or ionized metal plasma sputtering. In alternativeembodiments that use ALD, chemical precursors react with the surface ofa material one at a time to deposit a thin film on the surface. Inalternative embodiments that use GCIB deposition, a high-pressure gas isallowed to expand in a vacuum, subsequently condensing into clusters.The clusters can be ionized and directed onto a surface, providing ahighly anisotropic deposition.

Referring now to FIG. 2, a top-down view of a step in fabricating anend-stressed transistor device is shown. A hardmask 202 is deposited onthe uppermost layer of second semiconductor material 108. The hardmask202 is used to define fin regions, with material outside that covered bythe hardmask 202 being etched away to expose the underling insulatorlayer 104, producing fins 204. It is specifically contemplated that thisetch may be performed as part of a shallow trench isolation (STI)process, but it should be understood that other forms of anisotropicetching may be employed instead.

STI regions are formed by etching a trench in the substrate utilizing aconventional dry etching process such as reactive-ion etching (RIE) orplasma etching. RIE is a form of plasma etching in which during etchingthe surface to be etched is placed on a radio-frequency poweredelectrode. Moreover, during RIE the surface to be etched takes on apotential that accelerates the etching species extracted from plasmatoward the surface, in which the chemical etching reaction is takingplace in the direction normal to the surface. Other examples ofanisotropic etching that can be used at this point of the presentinvention include ion beam etching, plasma etching or laser ablation.Alternatively, fin structures 204 can be formed by spacer imagingtransfer processes.

While it is specifically contemplated that the hardmask 202 may beformed from silicon nitride, it should be understood that alternativehardmask materials may include silicon oxides, silicon oxynitrides,silicon carbides, silicon carbonitrides, etc. Spin-on dielectrics mayalso be utilized as a hardmask material including, but not limited to:silsequioxanes, siloxanes, and boron phosphate silicate glass (BPSG).The hardmask 202 may be patterned using photolithographic processes,where the hardmask 202 may be produced by applying a photoresist layerto the substrate 12 surface, exposing the photoresist layer to a patternof radiation, and then developing the pattern into the photoresist layerutilizing conventional resist developer.

It should be noted that the widths of the hardmasks 202 define the widthof fins 204. In particular, controlling the widths of the hardmasks 202determines whether the fins 204 form nanowires or nanosheets. It shouldbe understood that nanowires have a width-to-thickness ratio that isabout 2:1 or less, while nanosheets have a width-to-thickness ratio thatis about 2:1 or greater. Nanowires will be shown and discussed herein,but the present principles and processes apply with equal effect tonanosheets.

Referring now to FIG. 3, a step in fabricating an end-stressedtransistor device is shown. This figure shows a perspective view thatillustrates the spatial relationship between the fins 204 and alsodistinguishes the fins 204 from the hardmasks 202.

Referring now to FIG. 4, a step in fabricating an end-stressedtransistor device is shown. A dielectric material is filled in aroundthe fins 204 to a height that exceeds a height of the hardmask 202. Thedielectric material may then be polished down using, e.g., chemicalmechanical planarization (CMP) down to the height of the hardmask 202 toform dielectric fill 402. The dielectric fill 402 may be formed from anyappropriate dielectric, with oxides such as silicon dioxide beingspecifically contemplated.

CMP is performed using, e.g., a chemical or granular slurry andmechanical force to gradually remove upper layers of the device. Theslurry may be formulated to be unable to dissolve, for example, the workfunction metal layer material, resulting in the CMP process's inabilityto proceed any farther than that layer.

Referring now to FIG. 5, a step in fabricating an end-stressedtransistor device is shown. The fins 204 are cut to length according todesign requirements, forming sets of fins 502. The cuts form gaps 504.The fins 204 may be cut using any appropriate anisotropic etchincluding, for example, RIE.

Referring now to FIG. 6, a step in fabricating an end-stressedtransistor device is shown. A stress liner 602 is deposited to fill thegaps 504 at the ends of the fins 502. The stress liner material isfilled to a height equal to or greater than the height of the hardmasks202 and subsequently polished down to the height of the hardmasks 202using, e.g., CMP.

It should be noted that the stress liner 602 may be formed from amaterial that induces either compressive or tensile stress. A tensilestress inducing material may be a dielectric that generates a tensilestress on adjacent structures. In one specific example, a tensile stressinducing stress liner 602 may be formed from a tensile silicon nitrideand may be formed using, e.g., PECVD. The PECVD process can providenitride stress-inducing layers having an intrinsic tensile strain. Thestress state of the nitride stress including liners deposited by PECVDcan be controlled by changing the deposition conditions to alter thereaction rate within the deposition chamber. More specifically, thestress state of the deposited nitride strain inducing liner may be setby changing the deposition conditions such as: gas flow rate, pressure,radio frequency power, and electrode gap.

In another example, rapid thermal chemical vapor deposition (RTCVD) canprovide nitride tensile strain inducing liners 602 having an internaltensile strain. The magnitude of the internal tensile strain producedwithin the nitride tensile strain inducing liner 602 deposited by RTCVDcan be controlled by changing the deposition conditions. Morespecifically, the magnitude of the tensile strain within the nitridetensile strain inducing liner 602 may be set by changing depositionconditions such as: precursor composition, precursor flow rate, andtemperature.

Alternative tensile stress inducing materials may include, for example,aluminum oxides, hafnium oxides, zirconium oxides, hafnium siliconoxides, or any combination thereof.

A tensile stress inducing material may be a dielectric that generates atensile stress on adjacent structures. It is specifically contemplatedthat a compressive nitride may be used, but it should be understood thatalternative compressive stress inducing materials may include amorphouscarbon or a compressive oxide. A compressive strain inducing liner maybe formed, for example, using PECVD or high density plasma chemicalvapor deposition (HDPCVD). In one specific embodiment, PECVD can providenitride strain inducing liners having a compressive internal strain. Thestress state of the deposited nitride strain inducing liner may be setby changing the deposition conditions to alter the reaction rate withinthe deposition chamber, in which the deposition conditions include gasflow rate, pressure, radio frequency power, and electrode gap.

A silicon nitride stressed material having higher compressive stressvalues can be obtained by increasing radio frequency (RF) energybombardment to achieve higher film density by having more Si—N bonds inthe deposited material and reducing the density of Si—H and N—H bonds.Higher deposition temperatures and RF power improve the compressivestress levels of the deposited film. In addition, higher compressivestresses levels may be obtained in the deposited material at higherkinetic energy levels of plasma species. Bombardment of energetic plasmaspecies, such as plasma ions and neutrals, generates compressivestresses in the deposited material because film density increases.

Referring now to FIG. 7, a step in fabricating an end-stressedtransistor device is shown. The dielectric fill 402 is etched away usingany appropriate etching process. It is specifically contemplated that anisotropic wet etch may be used to selectively remove the dielectricmaterial, exposing gaps 702 between the fins 502.

Referring now to FIG. 8, a step in fabricating an end-stressedtransistor device is shown. The hardmask 202 is removed and the layersof first semiconductor material 107 in the fins 502 are selectivelyetched away, leaving nanowires 802 of second semiconductor material 108suspended from the stress liners 602. As noted above, nanosheets may beused instead of, or in addition to, nanowires 802.

Referring now to FIG. 9, a step in fabricating an end-stressedtransistor device is shown. The figure shows a cross-sectional view thatis cut along a plane in the middle of a set of nanowires 802. Dummygates 902 are deposited over and around the nanowires 802. It should beunderstood that final gates may be formed at this stage instead of dummygates 902, but the present embodiments focus on a replacement gateprocess. The dummy gates 902 may be formed from any appropriate materialincluding, e.g., polymorphous or amorphous silicon. The dummy gates 902may be formed by, e.g., depositing dummy gate material conformally overall surfaces, including the surfaces of the channels 802, using anyappropriate process, such as CVD, and anisotropically etching awaymaterial outside gate regions using, e.g., RIE. A liner 904 is formedover the dummy gates 902 and may be formed from silicon nitride or anyother appropriate dielectric material.

Referring now to FIG. 10, a step in fabricating an end-stressedtransistor device is shown. Source/drain regions 1002 are depositedbetween the dummy gates 902 and around the nanowires 802. Thesource/drain regions 1002 may be formed from the same semiconductormaterial as the nanowires 802 (e.g., the second semiconductor material108) or may be formed from a different material. It is specificallycontemplated that the source/drain regions 1002 may be epitaxiallygrown, but it should be understood that any appropriate depositionmethod may be used instead. The source/drain regions 1002 may be madeconductive through doping or silicidation.

As used herein, “p-type” refers to the addition of impurities to anintrinsic semiconductor that creates deficiencies of valence electrons.In a silicon-containing substrate, examples of p-type dopants, i.e.,impurities, include but are not limited to: boron, aluminum, gallium andindium. As used herein, “n-type” refers to the addition of impuritiesthat contributes free electrons to an intrinsic semiconductor In asilicon containing substrate examples of n-type dopants, i.e.,impurities, include but are not limited to antimony, arsenic andphosphorous. For doped source/drain regions, an dopant appropriate tothe device type is selected and either implanted into source/drainregions 1002 or added in situ during formation.

Alternatively, silicide formation involves depositing a refractory metalsuch as nickel or titanium onto the surface of the source/drain regions1002. Following deposition, the structure is then subjected to anannealing step using conventional processes such as, but not limited to,rapid thermal annealing. During thermal annealing, the deposited metalreacts with the semiconductor, forming a metal silicide.

Referring now to FIG. 11, a step in fabricating an end-stressedtransistor device is shown. The dummy gates 902 are etched away, leavinggaps 1102 over channel regions 1104 on the nanowires 802. The dummygates 902 may be removed using any appropriate etch including, e.g., awet chemical etch.

Referring now to FIG. 12, a step in fabricating an end-stressedtransistor device is shown. The dummy gates 902 are replaced with a gatestack. A layer 1202 of gate dielectric material is conformally depositedin the gaps 1102, with the gate dielectric layer 1202 being formed oninner surfaces of the liner 904 and on the channel regions 1104 ofnanowires 802 using, e.g., CVD. Gates 1204 are then deposited to fillthe gaps 1102.

The gate dielectric layer 1202 may be formed from any appropriateinsulating material, but it is specifically contemplated that the gatedielectric layer 1202 will be a high-k dielectric material. A high-kdielectric material is a material having a dielectric constant k that ishigher than that of silicon dioxide. Examples of high-k materialsinclude but are not limited to metal oxides such as hafnium oxide,hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide,lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide,zirconium silicon oxynitride, tantalum oxide, titanium oxide, bariumstrontium titanium oxide, barium titanium oxide, strontium titaniumoxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, andlead zinc niobate. The high-k may further include dopants such aslanthanum and aluminum.

The gates 1204 may be formed from any appropriate conductive materialincluding, e.g., metals such as tungsten, nickel, titanium, molybdenum,tantalum, copper, platinum, silver, gold, ruthenium, iridium, rhenium,rhodium, and alloys thereof, polysilicon, or a conducting metalliccompound material such as tantalum nitride, titanium nitride, tungstensilicide, tungsten nitride, ruthenium oxide, cobalt silicide, or nickelsilicide. The conductive material may further include dopants that areincorporated during or after deposition. Contacts may be formed toprovide electrical access to the gates 1204 and the source/drain regions1002 for operation of the transistor.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 13, a method for fabricating an end-stressedtransistor device is shown. Block 1302 forms a stack of alternatinglayers 107 and 108 of respective first and second semiconductormaterials by, e.g., epixtaxially growing layers of silicon germanium andsilicon. Block 1304 etches fins 204 out of the stack using hardmasks 202and any appropriate lithographic process.

Block 1306 deposits a dielectric fill 402 to fill in the spaces betweenfins 204. Block 1306 may include any appropriate deposition processfollowed by polishing the dielectric material down to a top level of thehardmasks 202. Block 1308 then uses an anisotropic etch to cut the fins204 to size, creating fins 502 and gap 504.

Block 1310 deposits a stress liner 602 on the ends of the fins 502. Thestress liner 602 may be compressive or tensile, depending on whether theend device will be a p-type device or an n-type device and it isspecifically contemplated that a silicon nitride liner material may beused.

Block 1312 etches away the dielectric fill 402 to expose the sides ofthe fins 502, leaving gaps 702. Block 1314 frees the nanowires 802 fromthe fins 502 by etching away the first semiconductor layers 107. Block902 deposits dummy gate 902 over and around the nanowires 802, althoughit should be noted that, in an alternative embodiment, a gate stack maybe deposited instead. Block 1318 deposits dummy gate liner 904 on thesurfaces of the dummy gate 902.

Block 1320 deposits source/drain regions 1002 around the dummy gates902. It is specifically contemplated that block 1320 grows thesource/drain regions 1002 epitaxially and with in situ doping. Block1322 then replaces the dummy gates 902 with a full gate stack, includinggate dielectric layer 1202 and gates 1204.

Having described preferred embodiments of integrated strained stackednanosheets FETs (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method for forming a semiconductor device,comprising: forming stress liners in contact with both ends of a fin ofalternating channel material and sacrificial material layers, whereinthe stress liners exert a stress on the fin; etching away thesacrificial material from the fin, such that the layers of the channelmaterial are suspended between the stress liners; and forming a gatestack on the suspended layers of channel material.
 2. The method ofclaim 1, further comprising forming a dielectric fill around the finbefore forming the stress liners.
 3. The method of claim 1, furthercomprising etching away the dielectric fill after forming the stressliners.
 4. The method of claim 1, wherein the channel material issilicon and wherein the sacrificial material is silicon germanium. 5.The method of claim 1, further comprising forming a dummy gate over andaround the suspended layers of channel material.
 6. The method of claim1, wherein forming the gate stack comprises replacing the dummy gatewith the gate stack.
 7. The method of claim 1, wherein the layers ofchannel material are nanowires.
 8. The method of claim 1, wherein thelayers of channel material are nanosheets.
 9. The method of claim 1,wherein the stress liners comprise silicon nitride.
 10. A method forforming a semiconductor device, comprising: forming a dielectric fillaround a fin of alternating channel material and sacrificial materiallayers; forming stress liners in contact with both ends of the fin afterforming the dielectric fill, wherein the stress liners exert a stress onthe fin; etching away the sacrificial material from the fin, such thatthe layers of the channel material are suspended between the stressliners; and forming a gate stack on the suspended layers of channelmaterial.
 11. The method of claim 10, further comprising etching awaythe dielectric fill after forming the stress liners.
 12. The method ofclaim 10, wherein the channel material is silicon and wherein thesacrificial material is silicon germanium.
 13. The method of claim 10,further comprising forming a dummy gate over and around the suspendedlayers of channel material.
 14. The method of claim 13, wherein formingthe gate stack comprises replacing the dummy gate with the gate stack.15. The method of claim 10, wherein the layers of channel material arenanowires.
 16. The method of claim 10, wherein the layers of channelmaterial are nanosheets.
 17. The method of claim 10, wherein the stressliners comprise silicon nitride.